Field of Invention
This invention relates generally to electromagnetic flowmeters, and more particularly to a flowmeter in whose excitation current for the electromagnetic coil is a low-frequency wave and in which the electrodes have a metallurgical hardness which minimizes the generation of a noise component when the liquid is a slurry having solid particles that impinge on the electrode surfaces.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects the field, a voltage is induced therein which is transferred to the electrodes. The voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Through A-C operation as disclosed in the Cushing U.S. Pat. No. 3,693,439 is clearly advantageous in that polarization is obviated and the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that normally are most troublesome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and PA1 2. induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow-induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undersirable signal that cannot readily be distinguished from the flow induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but elimination of its in-phase component has not been successful. Existing A-C operated electromagnet flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltage both with respect to phase and magnitude.
Hence it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In the patent to Mannherz et al., U. S. Pat. No. 3,783,687, whose entire disclosure is incorporated herein by reference, there is disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects. This low frequency wave is derived by means of a presettable scaler coupled to the standard a-c power line (60 Hz) and is at a frequency in the order of 17/8 , 33/4 , 71/2 or 15 Hz.
When the fluid being metered is in the form of a slurry containing solid particles which impinge on the surface of the electrodes as the slurry flows through the tube, we have found that a substantial noise component is generated which may make detection of the signal more difficult. Noise is any voltage that does not convey measurement information. Under the most favorable circumstances where noise has been minimized by filtering or other expedients, there are still certain sources of noise present resulting from the granular nature of matter and energy.
While noise fluctuations may be small compared with the total energy transfer involved in most measurements, the existence of a noise background limits the ultimate sensitivity to which a measurement can be carried. Sensitivity-limiting noise factors include Brownian motion, the Johnson noise in a resistance element and the Barkhausen effect in a magnetic element.
In the case of an electromagnetic flowmeter having a coil excited by an alternating current or a periodically interrupted direct current, it has been found that when the fluid being metered is a slurry containing solid particles, a spectrum of noise voltages is generated as a result of impingement of the solid particles on the surfaces of the electrodes. However, the frequency components of this spectrum does not include the frequency of the standard a-c power line (i.e. 50 or 60 Hz). Hence in a system of this type in which the excitation current is at the line frequency no difficulty is experienced in discriminating between the flow-induced signal and noise components to provide a favorable signal-to-noise ratio.
But in the case of an electromagnetic flowmeter of the above-described Mannherz type in which the excitation frequency is well below 60 Hz, we find that the frequency components of the noise spectrum do lie in the excitation frequency range; hence discrimination between noise and signal cannot be effected, and the signal-to-noise ratio is unfavorable. Indeed, in some instances, the magnitude of the noise relative to that of the flow-induced signal is such as to render the meter useless.